84 national geographic • march 2015
fish are better able to spot, and eat, the shrimp.
When light-up-on-disturbance life-forms
occur in large numbers—as they sometimes
do—moving through them can be like travel-
ing through a minefield of light. A fish moving
fast lights up like a shooting star; a boat creates a
bright, glowing wake. Any creature that doesn’t
want to be spotted would do better to avoid the
area altogether. Thus, even in the deepest, darkest
seas there’s an art to remaining hidden. Indeed,
most deep-sea animals have evolved to be black
or red, to stay out of sight if a burglar alarm goes
off. These colors also hide them from the search-
lights of deep-sea hunters, scanning the darkness
for prey. Although most bioluminescence is blue
or green, some of these hunters, such as the loose-
jaw dragonfish, use red light, which most deep-
sea animals can’t see.
The ROV is operated from a windowless
control room, with banks of screens facing a row
of seats ripped out of an old airplane. Watching
the screens is strangely hypnotic. The cameras
are high-definition and very clear—so you can
see creatures that are truly tiny, and in aston-
ishing detail. But most of the time all you see
is “marine snow”—particles of gunk gradually
sinking through the water. In the lights of the
vehicle, this looks like dust.
Every so often, however, an animal appears.
Perhaps a jellyfish. Or perhaps a small shrimp.
Or—wait! Wow! I almost choked on my cof-
fee. A fish has just appeared on the screen, one
I’ve read about but never seen. For the most
part, it looks like a regular fish. But attached
to its head, it has a long stalk and at the end of
the stalk, what looks like a fat, juicy, glowing
worm. But the worm is not a worm. It’s part of
the fish, which uses the “worm” as bait, tempting
the incautious and the hungry to their doom.
This is an anglerfish, one of the most voracious
predators of the deep. Unlike, say, sharks, which
chase down their victims, anglerfish are ambush
predators, enticing prey close by means of the
glowing lure, then pouncing. (Lures work be-
cause, thanks to the burglar alarm effect, many
creatures interpret light to mean food.)
In this case, the fish doesn’t make the light
itself. Instead, luminous bacteria, which live
within the lure, do the glowing. It’s of mutual
benefit: The bacteria get shelter, the fish gets
light. A similar arrangement is found in a few
other groups, but it is rare. Most luminous life-
forms make their own light.
To make light, you need three ingredients:
oxygen, a luciferin, and a luciferase. A luciferin
is any molecule that reacts with oxygen and in
doing so emits energy in the form of a photon—a
flash of light. A luciferase is a molecule that trig-
gers the reaction between oxygen and the lucif-
erin. In other words, the luciferin is the molecule
that lights up, while the luciferase is what makes
it happen. (In English, Lucifer is a name for Satan
before his fall from heaven; in Latin it means
“bringer of light.”)
Evolving to make light seems to be relatively
easy—it has happened independently in at least
40 different lineages. Perhaps that’s not surprising:
The ingredients are usually not hard to come by.
Plenty of substances can act as a luciferase. Stand
in the dark, mix egg white with oxygen and a lu-
ciferin from, say, a jellyfish, and you’ll probably
get a flicker of blue light. Moreover, in the ocean,
only those life-forms at the bottom of the food
chain must make luciferins. Everyone else can,
in principle, get them from diet: Thus, as humans
get vitamin C from eating oranges, some marine
animals get luciferins from eating a luminous
lunch. Which suggests the following possibility:
Luminous life is more common in the ocean in
part because the ingredients are easier to get.
Speaking of luminous lunch, here’s a weird
problem. As I mentioned, many animals that live
in the open ocean have evolved to be transpar-
ent, because this makes them harder to see. But
if you are transparent and you eat something
glowing, all of a sudden—oops—you are highly
visible. Which is why so many otherwise see-
through animals have guts that are opaque.
As the ROV resurfaces, people start to hurry
about. Any animals that have been captured are
rushed into cool rooms, so that they remain
comfortable while waiting to be examined. And
once again, it’s 10 p.m., and I’m standing in the